<script>
    const PI = 3.1415926;
    
    // 1、声明之后不允许改变    
 
    const PI = "3.1415926"

    PI = 3  // TypeError: Assignment to constant variable.

    // 2、一但声明必须初始化，否则会报错

    const MY_AGE  // SyntaxError: Missing initializer in const declaration
</script>